Conformational Dynamics of Planar and Twisted Excited States of Methoxy-Substituted Phenylpyrrolinium Salts

Abstract Arylpyrrolinium salts exhibit intriguing excited-state behavior that couples fluorescence with photochemical reactivity. While unsubstituted phenylpyrrolinium salts are nonfluorescent, methoxy-substituted derivatives display room-temperature fluorescence, providing an opportunity to investigate the structural and electronic factors governing their excited states. Here, we combine steady-state spectroscopy, time-resolved fluorescence measurements, and time-dependent density functional theory (TDDFT) calculations to establish a model for the photophysics of these compounds. The observed fluorescence originates from a predominantly planar S1 excited state that is in equilibrium with a lower-energy twisted conformer. The planar conformer corresponds to the bright emissive state, whereas the twisted conformer represents a dark state that was previously implicated as the reactive intermediate in photochemical [σ2+π2]-type fragmentative addition reactions with electron-deficient olefins. The biexponential fluorescence decay observed for selected derivatives is consistent with dynamic interconversion between these excited-state conformations. Further, an N-methyl analogue is nonfluorescent because steric effects destabilize the planar geometry required for emission, supporting the proposed model. Fluorescence quantum yields are low but strongly dependent on solvent and counteranion identity, demonstrating that environmental interactions modulate the equilibrium between emissive and nonemissive pathways. These results reveal how subtle structural modifications control the balance between fluorescence and photochemical reactivity in arylpyrrolinium salts and provide a framework for understanding excited-state conformational dynamics in this class of molecules.